1. Field of the Invention
This invention relates to an exposure mask and an exposure method and, more particularly, to a phase shift exposure mask and a method of phase shift exposure using the same. The exposure mask and exposure method according to the invention are applicable as a technique for forming various patterns; for instance they can be utilized in the exposure in semiconductor device manufacture processes for forming various resist patterns or like patterns.
2. Prior Art
In the manufacture of semiconductor devices or the like using photo-resist masks for pattern formation, there is a trend for finer processing dimensions. In these circumstances, in the technology of photolithography for obtaining semiconductor devices having fine structures, phase shift techniques are attracting attentions with an aim to improve the resolution The phase shift techniques are for improving the light intensity profile by providing a phase difference to light transmitted through a mask.
Among the prior art phase shift techniques are those disclosed in Japanese Patent Laid-Open Publication No. S58-173744, Marc D. Levenson et al "Improving Resolution in Photolithography with a Phase-Shifting Mask:", IEEE Transactions on Electron Devices, Vol. ED-29, No. 12, December 1982, p-p. 1828-1836, and Mark D. Levenson et al "The Phase-Shifting Mask I: Imaging Simulations and Submicrometer Resist Exposures", IEEE Transactions on Electron Devices, Vol. ED-31, No. 6, June 1984, p-p. 753-763.
Further, in Japanese Patent Publication No. s62-50811 is disclosed a phase shift mask, which has a predetermined pattern having a non-transparent portion and transparent portions on the opposite sides of the non-transparent portion, at least one of the transparent portions being provided with a phasing member to provide a phase difference between the opposite side transparent portions.
FIG. 1 illustrates a well-known phase shift technique called Levenson type. This technique will now be described. As shown in Fig. 1(a), on a transparent substrate 1, for instance a quartz substrate, a light-blocking pattern 10 is formed by using a light-blocking material, by using chromium (Cr) or like metal or a metal oxide, thus forming a line-and-space recurrence pattern as an exposure mask. The intensity distribution of light transmitted through the exposure mask is as shown by a plot A1 in Fig. 1(a), ideally being zero in correspondence to the light-blocking pattern 10 and at a certain level in corresponding to the other portions (i.e., transmitting portions 12a, 12b). With respect to the transmitting portion 12a, for instance, the transmitted light given to the exposed material has an intensity distribution as shown by plot A2 in Fig. 1(a), having hill-like maxima which are produced by diffraction of light or like cause in opposite side foot portions. The transmitted light A2' in the transmitting portion 12b is shown by a dashed plot. The resultant light transmitted through the transmitting portions 12a and 12bas shown by plot A3, has an intensity distribution lacking in sharpness. Dimness of image thus results from the diffraction of light, and therefore, it is impossible to attain sharp exposure. To cope with this drawback, it is thought to provide a phase shifter 11a (commonly called shifter and formed by using SiO.sub.2, a resist or like material) on every other one of the transmitting portions 12a, 12b in the recurring pattern. With this arrangement, the dimness of image due to the diffraction of light is canceled by phase inversion, as shown in Fig. 1(b). Thus, it is possible to obtain transfer of sharp image and improve the resolution and focal point redundancy. More specifically, if the phase shifter 11a formed on the transmitting portion 12b can provide a phase shift by 180.degree., for instance, light transmitted through the phase shifter 11a is inverted as shown by plot B1. On the other hand, light from the adjacent transmitting portion 12b is not transmitted through any phase shifter and hence is not inverted. The light which reaches the exposed material, that which is inverted and that which is not, cancel each other in the foot portions of their intensity distributions, as shown by plot B2. Consequently, the intensity distribution of light which reaches the exposed material has an ideal sharp form as shown by plot B3 in Fig. 1(b).
In this case, to make the effect most reliable it is most advantageous to cause 180.degree. phase inversion. To this end, the phase shifter 11a is formed as a film with a thickness D of EQU D=.lambda./(2(n-1)
where n is the refractive index of the phase shifter material, and .lambda. is the exposure wavelength.
In forming a pattern through exposure, a pattern produced by contracted scale production is called a reticle, while that which is produced by equal scale production is called a mask. Alternatively, what corresponds to an original is called a reticle, and its duplicate is called a mask. According to the invention, the masks and reticles which are used with various senses as noted above are generally referred to as masks.
The phase shift mask as noted above, in which the phase of light is shifted (ideally subjected to 180.degree. inversion) between adjacent light transmitting portions), is referred to as spacial frequency modulation type (or Levenson type). There are various other shift masks including an edge emphasis type, a light-blocking effect emphasis type, etc. Among these phase hift masks are those without any light-blocking portion ((such as those called chromium-less type). In any of these phase shift masks, the portion for transmitting exposure light is at least locally provided with a phase shifter for causing a phase shift.
The above technology of the phase shift mask utilization is very effective for line-and-space or like recurrence patterns such as shown in FIG. 5. However, it presents inconvenience in the case of forming a non-recurrent independent pattern.
More specifically, in the phase shift technique a phase difference is given light portions for exposure for adjacent patterns, thereby utilizing an effect of cancellation of the two light intensity distributions. When forming an independent line or a contact hole, there are no light portions in the proximity of one another. In such a case, therefore, it is impossible to obtain direct realization of the phase shift technique.
As shown in FIG. 6, therefore, it has been necessary to provide, in addition to a transmitting portion 12 (with a phase shift of 0.degree.), which is provided in correspondence to the pattern to be formed such as to transmit exposure light without phase shift, phase shifters 11 (with a phase shift amount of 180.degree., for instance) in the vicinity of the transmitting portion 12 (see Terasawa et al, The 49-th symposium of the Japan Society of Applied Physics, Autumn 1988, No. 2, p. 497, 4a-K-7).
In the prior art as above, it is necessary to provide in the light-blocking portion 10 a main space for forming the transmitting portion 12 for pattern formation and sub-spaces for forming the phase shifters 11. In the independent line pattern formation mask shown in FIG. 2(a), such phase shifters 11 are formed along and in the vicinity of the opposite sides of a longitudinally elongate transmitting portion 12 as the main space. In the hole pattern formation mask as show in FIG. 2(b), phase shifters 11 are formed in the vicinity of and along four sides of a rectangular transmitting portion 12. Particularly, the mask pattern shown in FIG. 2(b) is considerably effective in contact hole formation using a positive resist.
The method of using such an exposure mask, in which auxiliary transmitting portions (called sub-shifters) are formed along the sides of the central pattern such that they are shifted in phase with respect to the central pattern, is called a sub-shifter type phase shift method.
The sub-shifter type phase shift method is effective for such purpose as increasing the resolution of independent patterns. This will be described hereinafter in detail. The method is typically shown in FIG. 3. As shown, along the four sides of a central pattern C phase shifters (i.e., sub-shifters) are provided as edge patterns S1 to S4 to elevate the contrast of the pattern. There are of course optimum positions of disposition and dimensions of the sub-shifters. In the case of i-beam exposure light, for instance, for a square central pattern C (hole pattern) with one side length L.sub.3 of 0.40 micron, the length L.sub.1 and width L.sub.4 of each sub-shifter are 0.45 and 0.20 micron, respectively, and the distance L.sub.2 thereof with respect to the corresponding side of the central pattern is 0.575 micron.
As is seen from FIG. 3, by arranging the sub-shifters along the four sides of the central pattern the size of the overall pattern is increased. This means that where there are two or more such pattern groups, a limitation is imposed on the distance, at which two contact holes can be close to each other. FIG. 4 shows an example, in which two contact holes (central patterns C1 and C2) are too close to each other. In this case, adjacent sub-shifters S2 and S2' among the sub-shifters S1 to S4 and S1' to S4' are merged together to form a peak. Labeled D is the center-to-center distance between the two contact holes (central patterns C1 and C2). The merged peak has a higher intensity than those of the outer sub-shifters. This is so because the inner sub-shifters are influenced by diffracted light form the opposite side contact holes, while the other sub-shifters receive influence of only a single associated contact hole. Further, the intensity of the main peak is reduced by the influence of proximity. These phenomena will naturally pose future problems in attempts to further increase the LSI integration density.
As noted above, in the sub-shifter type phase shift method it is necessary to dispose sub-shifters in an optimum distance relation to the central pattern. For example, when a contact hole of 0.4 micron is to be formed by using i beam and under conditions of
=365 nm.lambda., PA1 NA=0.50, PA1 &lt;=0.3 and PA1 Focus=Best Focus,
it is necessary to dispose each sub-shifter at a position of 575 microns from the hole center.
FIG. 5 shows light intensity distributions obtained by simulation with and without sub-shifters. The solid curve represents the case with sub-shifters, and the dashed curve represents the case without sub-shifters. The calculations were made with the length and width of sub-shifters set to 0.45 and 0.20 microns, respectively. As is obvious from the graph, the provision of the sub-shifters has an effect of greatly improving the contrast.
FIG. 5 shows the result of simulation carried out under the above conditions with the center-to-center distance D between two contact holes set to 1.6 microns. The adjacent inner sub-shifters are merged together to form a peak. The intensity of the merged peak is as high as 0.31 in comparison to an outer peak intensity of 0.23.
FIG. 6 shows light intensity distributions in cases of D=2.0 micron and D=1.6 micron. A slight main peak intensity fall from 0.8 to 0.78 micron will be seen, which is due to the influence of the distance reduction. When the distance is reduced down to D=1.4 micron, the peak intensity provided by the sub-shifters is increased up to 0.71 micron, as shown in FIG. 7. This is substantially the same as in the claim, in which three contact holes are formed by using the Levenson type phase shift method.